Electronic components, particularly microelectronic components such as semiconductor devices (chips), often have a plurality of terminals (also referred to as bond pads, electrodes, or conductive areas). In order to assemble such devices into a useful system (or subsystem), a number of individual devices must be electrically interconnected with one another, typically through the intermediary of a printed circuit (or wiring) board (PCB, PWB).
Semiconductor devices are typically disposed within a semiconductor package having a plurality of external connection points in the form of pins, pads, leads, solder balls, and the like. Many types of semiconductor packages are known, and techniques for connecting the semiconductor device within the package include bond wires, tape-automated bonding (TAB) and the like. In some cases, a semiconductor device is provided with raised bump contacts, and is connected by flip-chip techniques onto another electronic component.
Generally, interconnections between electronic components can be classified into the two broad categories of “relatively permanent” and “readily demountable”.
An example of a “relatively permanent” connection is a solder joint. Once two components are soldered to one another, a process of unsoldering must be used to separate the components. A wire bond is another example of a “relatively permanent” connection.
An example of a “readily demountable” connection is rigid pins of one electronic component being received by resilient socket elements of another electronic component. The socket elements exert a contact force (pressure) on the pins in an amount sufficient to ensure a reliable electrical connection therebetween. Interconnection elements intended to make pressure contact with an electronic component are referred to herein as “springs” or “spring elements”.
Spring elements are well known, and appear in a variety of shapes and sizes. In today's microelectronic environment, there is a profound need for all interconnection elements, including springs, to become smaller and smaller, in order that a large plurality of such interconnection elements can be disposed in a small area, to effect a high density of interconnections to electronic components.
Prior art techniques for making spring elements generally involve stamping (punching) or etching a spring material, such as phosphor bronze or beryllium copper or steel or a nickel-iron-cobalt (e.g., kovar) alloy, to form individual spring elements, shaping the spring elements to have a spring shape (e.g., arcuate, etc.), plating the spring elements with a good contact material (e.g., a noble metal such as gold, which will exhibit low contact resistance when contacting a like material), and molding a plurality of such shaped, plated spring elements into a linear, a peripheral or an array pattern. When plating gold onto the aforementioned materials, sometimes a thin (for example, 30-50 microinches), barrier layer of nickel is appropriate.
Various problems and limitations are inherent with such techniques of making spring elements.
For example, these processes are limited when applications demand that a plurality of springs (interconnection elements) be arranged at a fine (e.g., 10 mil) pitch. Such a fine pitch inherently demands that each spring be sized (i.e., in cross-section) substantially smaller (e.g., 3 mil) than the pitch. A punch-out area must be accommodated, and will limit how much material is left over to form springs. At best, even through it may be relatively straightforward to punch out springs as small as 1 mil, such small sizes impose limitations on the contact force that can reliably be exerted by the springs. This is especially poignant in the context of fabricating area arrays of springs.
Generally, a certain minimum contact force is desired to effect reliable pressure contact to electronic components (e.g., to terminals on electronic components). For example, a contact (load) force of approximately 15 grams (including as little as 2 grams or less and as much as 150 grams or more, per contact) may be desired to ensure that a reliable electrical connection is made to a terminal of an electronic component which may be contaminated with films on its surface, or which has corrosion or oxidation products on its surface. The minimum contact force required of each spring demands either that the yield strength of the spring material or that the size of the spring element are increased. As a general proposition, the higher the yield strength of a material, the more difficult it will be to work with (e.g., punch, bend, etc.). And the desire to make springs smaller essentially rules out making them larger in cross-section.
Another limitation attendant prior art interconnection elements is that when hard materials (such as would be used for making springs) are employed, relatively “hostile” (e.g., high temperature) processes such as brazing are required to mount the interconnection elements to terminals of an electronic component. For example, it is known to braze rigid pins to relatively “durable” semiconductor packages. Such “hostile” processes are generally not desirable (and often not feasible) in the context of certain relatively “fragile” electronic components such as semiconductor devices. In contrast thereto, wire bonding is an example of a relatively “friendly” processes which is much less potentially damaging to fragile electronic components than brazing. Soldering is another example of a relatively “friendly”, process.
Another problem associated with mounting springs on electronic components is largely mechanical in nature. In cases where a spring is mounted at one end to a substrate (which, for purposes of this proposition is considered to be an immovable object), and is required to react forces applied at its free end, the “weak link” (weakest point, in service) will often be the point at which the spring is attached (e.g., the base of the spring is bonded) to the substrate (e.g., terminal of an electronic component). This accounts, at least in part, for the requirement to employ “hostile” processes (e.g., brazing) to mount the springs to the substrate.
Another subtle problem associated with interconnection elements, including spring contacts, is that, often, the terminals of an electronic component are not perfectly coplanar. Interconnection elements lacking in some mechanism incorporated therewith for accommodating these “tolerances” (gross non-planarities) will be hard pressed to make consistent contact pressure contact with the terminals of the electronic component.
In many modern electronic systems, one or more packaged semiconductor devices are mounted to circuit boards. Various packaging types are well known. Generally, all semiconductor packages have external connections which are either pins, pads, leads, ball bumps, or the like.
One type of semiconductor package is typified by U.S. Pat. No. 4,700,276 (“FREYMAN”), entitled ULTRA HIGH DENSITY PAD ARRAY CHIP CARRIER. As generally disclosed therein, a ceramic substrate is provided with a plurality of through holes plugged with solder on its bottom surface. These solder plugs (206) are arranged in an array pattern, and form external surface mount interconnection points for the final chip carrier arrangement. The solder plugs are generally hemispherical, and permit the substrate to sit high above the board to which the carrier is mounted. A semiconductor package having an array of solder balls as its interconnection points on an external surface thereof is referred to herein as a Ball Grid Array (BGA) type package.
Generally, BGA solder balls are of two types: (1) eutectic masses that melt upon reflow; and (2) masses such as of 90:10 lead:tin that are not melted, but rather are attached with a eutectic material. The first type of solder ball will collapse slightly (e.g., approximately 6 mils) upon reflow, resulting in some concern over the final planarity of the plurality of connections effected thereby. The second type of solder ball does not collapse, since they are not reflowed. However, since a eutectic material is employed to attach the second type of solder balls, certain substrate materials that cannot withstand the heat associated with eutectic attach processes cannot be employed. This information is provided for general background purposes.
Another type of semiconductor package is the Land Grid Array (LGA), which is provided with a plurality (e.g., an array) of terminals (contact pads (or “lands”) on a surface thereof. Generally, resilient interconnection elements are used to make electrical connections to the lands of an LGA. The present invention discloses a “socket” having a plurality of resilient interconnection elements for making electrical connections to the terminals of an electronic component such as an LGA-type semiconductor package.
It is generally desired that sockets for LGA and BGA type semiconductor packages be soldered down (e.g., surface-mounted) to a circuit board. Prior art sockets relying on pins require corresponding holes through the circuit board. Using conventional techniques of fabricating holes (e.g., plated through holes) in circuit boards, spacing between adjacent holes (pitch) is typically constrained to no less than 100 mils between adjacent holes. Moreover, plated through holes represent an additional cost in the manufacture of circuit boards. What is needed is a “solder-down” or “surface-mountable” socket to permit connections to be made at a finer pitch (e.g., 50 mils) and at reduced cost.
Additional references of interest, vis-a-vis BGA and LGA type packages include the following U.S. Pat. Nos. 5,241,133; 5,136,366; 5,077,633; 5,006,673; and 4,700,473.
The aforementioned BGA type package is surface-mounted, by soldering the semiconductor package down onto a PCB. This effects a more-or-less permanent connection of the packaged semiconductor device to the PCB. In order to remove the packaged semiconductor device (such as for replacement or upgrading), it would be necessary to unsolder the entire package from the PCB—a process which can damage either the PCB or the semiconductor device contained within the semiconductor package. Moreover, in order to unsolder a component from a PCB, it is generally necessary to remove the PCB from the system in which it is located.
Techniques for demountably connecting semiconductor packages to PCBs do not suffer from such vagaries. For example, a semiconductor package having pins is readily plugged into a socket which is permanently mounted to a PCB, and is just as readily removed from the socket.
One aspect of the present invention is directed to providing a technique whereby any electronic component such as a BGA or an LGA type semiconductor package can readily be demounted, without unsoldering, from a PCB—in other words, providing “sockets” for BGA and LGA type semiconductor packages. This facilitates not only the replacement/upgrading of the packaged semiconductor device, but also provides the opportunity to test the packaged semiconductor device in instances where the PCB is a probe card, or a probe card insert.
As a general proposition, demountable connections require some sort of pressure contact to be made between electronic components. Sockets for receiving pinned semiconductor packages typically have leaf-type spring elements for receiving the package pins.
The following U.S. Pat. Nos. are cited as being of interest: U.S. Pat. Nos. 5,386,344; 5,336,380; 5,317,479; 5,086,337; 5,067,007; 4,989,069; 4,893,172; 4,793,814; 4,777,564; 4,764,848; 4,667,219; 4,642,889; 4,330,165; 4,295,700; 4,067,104; 3,795,037; 3,616,532; and 3,509,270.
Another aspect of the present invention is directed to techniques for forming solder balls and/or raised solder bumps on electronic components, particularly on chip carriers or semiconductor packages. In the main hereinafter, techniques for forming solder “balls” are discussed.
Techniques for forming solder balls and/or raised solder bumps on electronic components include, by way of example only:                (1) applying dollops (small quantities) of solder paste to contact pads and reflowing the solder paste;        (2) solder-plugging plated areas (see, e.g., FIG. 2c of FREYMAN);        (3) molding solder ball contacts directly on a substrate (see, e.g., U.S. Pat. No. 5,381,848); and        (4) filling holes in a film carrier with solder, placing the carrier over the substrate, and reflowing the solder to adhere to contact pads on the substrate (see, e.g., U.S. Pat. No. 5,388,327).        
Other methods of forming raised solder contacts, of some relevance to the present invention, are the techniques disclosed in the aforementioned commonly-owned, copending U.S. patent application Ser. Nos. 08/152,812, 08/340,144 and 08/452,255, which generally involve bonding a wire at two (both) ends to a terminal of an electronic component and overcoating the wire with solder. (See, e.g., FIGS. 24A and 24B of Ser. No. 08/452,255; FIG. 16 of Ser. No. 08/340,144; and FIGS. 2-5 of Ser. No. 08/152,812.)